It is known in the prior art to control the operation of a passenger vehicle propulsion motor control circuit with a programmed microprocessor coupled to a motor control chopper apparatus as shown in U.S. Pat. No. 4,282,466, the disclosure of which is incorporated herein by reference. The chopper frequency cycle of operation can be controlled by a crystal controlled interrupt clock as disclosed in U.S. Pat. No. 4,339,697. The frequency of the chopper operation is selected to reduce generated noise disturbance of the vehicle detection and speed control signals received from track signal blocks which are described in an article published in the Westinghouse Engineer for September 1972 at pp. 145-151.
It is desired that the chopper frequency cycle be accurately controlled by clock signals in relation to the particular track signal frequencies utilized for the vehicle speed control and detection to prevent interference with the operation of the track signaling apparatus. Even though the chopper frequency interrupt clock is a crystal controlled oscillator with a frequency selected not to generate harmonics which could be set by the track signaling filter circuits, it is desired to detect any frequency shift of the chopper interrupt clock before the resulting frequency change becomes a problem in relation to vehicle speed control and signal block occupancy detection.